# Oil Drop Electric Field Problem

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1. Nov 11, 2014

### yasemonkey

1. The problem statement, all variables and given/known data
An oil drop is charged negatively. How much charge is on the drop if the Electric Field is 6,400 N/C at a distance of 1.2 m?

2. Relevant equations
E=k q/r^2
I don't know if I should be using that equation for this one.

3. The attempt at a solution
I tried to do E/(k x r^2) = q but that didn't work
I also tried E/r^2= q but that also didn't work

I know the answer has to be 1.3 x 10^3

Thanks in advance for all the help!

2. Nov 11, 2014

### Orodruin

Staff Emeritus
You have the correct equation for field strength, you are just not solving correctly for q.

A good check is whether or not your resulting expression has the correct physical units.

3. Nov 11, 2014

### yasemonkey

hmm... okay

and I'm right in saying k= 9 x 10^9

4. Nov 11, 2014

### Orodruin

Staff Emeritus
No. As many other constants, k is dimensionful and k = 9 x 10^9 N m^2/C^2 - units are important and the units of your final answer must also make sense.

5. Nov 11, 2014

### yasemonkey

Oh I have to square root k x r^2 !